FIG. 1 is a reduced complexity illustration of a typical buck mode DC—DC converter, having an upper switch SWU and a lower switch SWL, each of which is customarily implemented as a field effect transistor, coupled in series between a source of input voltage Vin and a reference terminal (e.g., ground). An inductor is coupled between the common node 10 between the two switches (node 10 often referred to as the phase node) and an output node 20 to which a capacitor C is coupled. In the schematic illustration of FIG. 1, the inductor has been shown as a parasitic resistor DCR (direct current resistance) in series with an inductor L. Respective upper gate drive and lower gate drive signals are applied to the gates of the MOSFETs of which the upper and lower switches SWU and SWL are respectively configured, so that the switches are turned on and off in a complementary manner.
In order to achieve certain control related to inductor or load current and provide over-current protection for the converter, current information is required. This has been traditionally obtained by sensing the voltage or current of the components connected to the phase node, such as the lower switch, the upper switch and the inductor. On conventional implementation for this purpose is diagrammatically illustrated in FIG. 2, which has series-coupled resistor Rs and capacitor Cs connected in parallel with the inductor L. The R and C construct a low pass filter which will filter out the ac voltage. And the voltage across the capacitor will reflect the current information through the inductor L. Namely, this voltage can be used to measure inductor current. Typically, however, the value of this voltage is only in the millivolt range as the DCR is generally measured in milliohm range.